The present invention relates to a continuously operable sugar centrifugal. More specifically the invention relates to a method of improving the purity of sugar produced by continuously operable sugar centrifugals.
Continuously operable sugar centrifugals have been employed for a long time heretofore. Due to their comparatively simple construction, and in particular their specifically energy saving operation, they are highly superior to batch centrifugals. Heretofore, however, the sugar industry has not been able to do without batch centrifugals, especially when sugar of a high purity is to be produced, or when by just one centrifuging operation the purity of the sugar shall be improved to a large degree.
It has been known for a long time that the sugar-technological phenomena in washing are responsible for the reduced purity of sugar produced by continuously operating centrifugals. It was assumed that in a batch centrifugal the washing effect is enhanced by the fact that, compared to a continuously operating centrifugal, the sugar is washed in a comparatively thick and dense layer. The intensity and the time of contact between the wash liquid and the sugar crystals were thus thought to be optimal. Following this assumption, so-called continuously operating thick-layer flow centrifugals were developed. But the result was disappointing. A substantial increase in sugar purity could not be achieved. Then tests were made by accumulating the sugar in these so-called continuous centrifugals so as to provide dense crystal packing conditions for the washing similar to those in batch centrifugals. But again, the sugar purity could not be improved to such a degree that it could be compared to the purity of sugar produced in batch centrifugals.
At a much earlier stage of development efforts had been made to increase the application of wash liquid in continuous centrifugals. But this measure met with the sugar industry's objection just because this increased the quantity of runoff to be re-processed, especially by energy-consuming re-evaporation. Besides, the results were suprisingly negative, since this measure could not considerably improve the purity of the sugar. Instead, the increasing quantity of wash liquid went along with increasing sugar losses resulting from partial dissolution of sugar crystals.
In the centrifugal according to German Pat. No. 65,118 the upper part of the conically shaped basket is covered by an apron made of resilient material. This apron rotates with the basket. The apron is intended to bring the wash liquid into a more intimate contact with the medium to be centrifuged. As a result, the contaminated wash liquid mist is applied again to the medium. A shielding effect as provided by the invention has neither been intended nor realized in this reference.
Though German Patent Application No. 2,447,175 reveals the realization that any wash liquid mist entrained outside of the washing zone is detrimental to the sugar, because it carries moisture into those areas where the sugar shall become or remain dry, it does not reveal the essential realization that the entrained wash liquid mist is responsible for the comparatively low increase in the purity of sugar produced in a continuously operating centrifugal. Moreover, this prior art offers an unpractical and technologically disadvantageous solution to the problem of a continuous high purity sugar production, since it suggests to seal the washing zone as a chamber from the other inner space of the centrifugal, especially the inner space of the basket. Such sealing can be accomplished only if suitable sealing elements rest at a sufficiently high pressure on the sugar layer moving across the inner surface of the basket or its separating screen. Such measures impair an unobstructed flow of the sugar. As the sugar layer never has a sufficiently uniform thickness, this produces either non-dense spots or undesired obstacles to the flow of the sugar. Not the least problems are caused by the necessary sealing elements because they are subject to heavy wear by the sugar passing under them. Further, sugar crystals are damaged by abrasion at the same time. Therefore, this prior art has substantial disadvantages in actual operation. In a chamber-like sealing of the washing zone, contaminated wash liquid mist is forced back to the sugar, the sugar is again contaminated. The fractions of a second or the few seconds the sugar takes to pass through the washing or drying zone or through the entire machine are too short a time to eliminate this re-contamination of the drying sugar.